Based on the principle of soil water balance, the change of soil water content during the whole growth period of eggplant with the variety “Angela” as the material is simulated through the test of greenhouse environmental factors. The crop coefficient is determined by the optimization method. The evapotranspiration of eggplant is simulated under the condition of drip irrigation under mulch film. The results showed that the simulated values of soil water content in the growth period of eggplant are in good agreement with the measured values, and the relative error is less than 10%. The variation rule of the crop coefficient and eggplant leaf area index are consistent. The crop coefficient in the early increases gradually, in the vigorous growth period of crop coefficient reaches the maximum value of 0.518, then began to decreases from 0.518 reduced to 0.505 and then increased gradually. The fluctuation is mainly affected by pruning management. The change of water requirement of eggplant in greenhouse is smaller in the early stage of growth between 0.2–2.4 mm/d. And the change in the late of the growth period is larger between 0.1–3.1 mm/d. The accumulated value of evaporation and transpiration increases gradually, and the highest value is 290 mm/d.